Method of coating substrates by vapor deposition

ABSTRACT

A method of coating substrates by vapor deposition. The material to be deposited is taken up in a lacelike body built up from graphite yarn heated to the vaporization temperature by the passage of electrical current.

United States Patent [72] Inventor Johannes Jacobus Asuerus Ploos Van[50] Field of Search 1 17/106,

Amstel 107,46 CC,46 CB,46 CA; l18/48,49,49.1,49.5; Emmasingel,Eindhoven, Netherlands 2 19/273, 274, 275 [211 App]. No. 765,826 [22]Filed Oct. 8, 1968 Rdel'ellces Clled [45] Patented Sept. 21, 1971 UNITEDSTATES PATENTS [7 1 Assignee Phillips orporation 2,665,225 1 1954 Godley117/107 New 2,665,227 1/1954 Clough 6t 61.... 117/107 1 Pflomy 0t.l0,1967 3,350,219 10/1967 Shaler 118/49.1 (x 1 [33] Netherlands [31]6713713 Primary Exammer-Alfred L. Leavltt Assistant Examiner-William E.Ball W Attorney-Frank R. Trifari [54] METHOD OF COATING SUBSTRATES BYVAPOR DEPOSITION 6 Chums 3 Drawing Figs ABSTRACT: A method of coatingsubstrates by vapor deposi- [52] U.S.Cl 117/106, tion. The material tobe deposited is taken up in a lacelike 117/107, 219/274, 118/491,118/495 body built up from graphite yarn heated to the vaporization [51Int. Cl C23c13/12 temperature by the passage of electrical current :1 II I J 9 Z f;

PATENIED saw a SHEET 1 0F 2 FIG.2

INVENTOR. JOHANNES J.A.PLOOS VAN AMSTEL METHOD OF COATING SUBSTRATES BYVAPOR DEPOSITION The invention relates to the coating of substrates byvapor deposition.

As is known, this is generally carried out in an atmosphere which isinert relative to the substance to be deposited in which, in order tocheck impediment of the vapor molecules on their way to the substrate,the gas pressure is chosen to be low and the method is usually carriedout in a vacuum.

The vaporization of the substance to be deposited is usually effected bymeans of a vaporizer element which is heated by the passage of current.

According to some of the known methods, the substance is caused tovaporize on the surface of a heating element. A drawback hereof is thatthe substance can be present only in restricted quantities and hence thevapor deposition of thick layers or the continuous vapor deposition isnot possible.

In other known methods this drawback is avoided by placing a largequantity of the substance to be vaporized directly or in a containerinside a heating element which has the form of a helically woundfilamentary body. However, a shadow effect of the heating body alwaysoccurs which hampers the obtaining of an even coating.

The use of a vaporizing element is also known already to which thesubstance to be vapor-deposited is applied continuously from a containerduring the vaporization process (American Pat. Spec. 2,665,227). In thismethod, an approximately horizontally arranged vaporizer element in theform of a rod is used which is provided with a channellike grooveextending in the longitudinal direction in which the substance isallowed to flow from the container. This element is composed of amaterial which is readily wetted by the substance to be vapor-depositedso that it is coated with the substance, all over its surface. For manyapplications graphite was found to be suitable, if desired, convertedsuperficially into a high-melting-point metal carbide.

A drawback of such vaporizer elements is that during use they have to bearranged approximately horizontally. In addition, the layer thickness ofthe substance to be vapor deposited in the groove is different from thaton the further surface of the element. The result of this is adifference in vaporization at the area of the groove so that substratesarranged around the element are coated in different thicknessesdependent upon their position.

The invention is based on the recognition of the fact that theabove-mentioned drawbacks can be avoided by using an electricallyconducting vaporizer element which is built up from a porous material.

According to the invention, a vaporizer element is used for thispurpose, which is constructed from a lacelike body of graphite yarn.

Such a material is commercially available and may be obtained, forexample, by carbonizing rayon yarns, whether or not woven, knitted ortwined, at temperatures above approximately 2000" C. for example anArticle by MoLindsey in Design Engineering of 6 Apr. 1965 titled:Developments in Carbon and graphite Textiles f.e. Materials whichadvantageously can be used are told by Morganite Research andDevelopment Ltd. UK. as graphite card grade 6301 G and 6303 G.

The invention relates to a method of coating substrates byvapor-deposition from the surface of a vaporizer element built up fromgraphite and heated by the passage of current, characterized in that thesubstance to be vapor-deposited is taken up in a vaporizer elementconsisting of a lace-body which is built up from graphite yarn and isthen heated at a temperature at which said substance vaporizes.

In this manner a very even coating of the whole vaporizer surface can beobtained, which is a condition for obtaining an even deposition onsubstrates arranged around the vaporizer. Any shadow effect of thematerial of which the element consists can be excluded by the finenessof the pores.

In connection herewith a vaporizer element is preferably used which isbuilt up from yarns, the elementary strands of which have a thickness ofunder 20 microns.

The substance to be vaporized may be incorporated in the vaporizerelement in comparatively large quantities. If very thick layers have tobe vapor-deposited, or large numbers of substrates have to be coated ina continuous process however, it is of advantage to add a furtherquantity of the substance to the element during the vaporizationprocess.

According to a useful embodiment of the invention a porous lacelike bodyis used which comprises a coaxial cavity. As a result of this it ispossible to incorporate larger quantities of the substance to bevapor-deposited in the vaporizer.

Some substances, for example, copper, germanium and tim, do not reactwith the graphite material of the vaporizer. If such a substance is madeto melt in the vaporizer, it penetrates through the pores in the wall tothe outer surface on which an even layer is then formed. In this case itis recommendable to use a hollow vaporizer element which can easily befilled with the material to be vaporized.

With substances, for example, chromium, which poorly wet graphite, it isof advantage, in order to obtain an even vaporization, to convert thegraphite entirely or superficially into metal carbide. As is known, manyof these carbides can withstand the influence of very high temperaturesand are more easily wetted by various substances. For this purpose areto be considered the carbides of metals of the promps IVa, Va and Vla ofthe periodic system of elements, particularly zirconium carbide, and inaddition aluminum carbide.

However, cases may also present themselves in which the graphite and thesubstance to be vapor-deposited react with each other while formingcarbides. This is the case notably in the above-mentionedcarbide-forming metals. In itself this reaction is by no meansobjectionable because the carbide vaporizes at higher temperatures thanthe metals from which they are formed. If the carbides decompose at thevaporization temperature of the material, the vaporization may becontinued after no free metal is present any longer in the vaporizer.The metal bound in carbide is then also vaporized and a graphiteskeleton remains. This is significantly the case, for example withaluminum carbide.

Of course it is possible with these metals which do react with graphite,to convert the graphite body previously entirely or superficially into acarbide of a metal other than the metal which is to be vaporized. Forexample, the graphite may be converted into zirconium carbide, as aresult of which an element of a very large strength is obtained, whichcan be used repeatedly for vapor-deposition of metals, for example,cobalt and chromium.

The supply to the vaporizer of the substance to be vapordeposited may beeffected in a liquid and in a solid state, both prior to and during thevaporization.

According to a further aspect of the invention a device for coatingsubstrates by vapor-deposition comprises a container in which thesubstance to be vaporized can be molten and from which the moltensubstance flows into a vaporizer element consisting of a lacelike bodywhich is built up from yarns of graphite and/or a carbide. For thispurpose the action of gravity and/or capillary action occuring may beused.

In some cases, for example, when silicon or germanium are supplied to anelement which is'converted into silicon carbide, such a strong capillaryaction occurs that, if such an element is placed in the molten materialwith one extremity this will be sucked up into it.

For the filling with metal in the solid-state it is of courserecommendable to use a vaporizer element which is provided with acoaxial cavity. The metal in the form of powder, grains or wire, caneasily be provided in said cavity. For the continuous supply during thevaporization process, metal in the form of a wire is most suitable. Thisis the case also when replenishing the molten substance which issupplied to the vaporizer element through a container.

In order that the invention may be readily carried into effect, a fewexamples thereof will now be described in greater detail, with referenceto the accompanying drawing in which FIG. 1 shows in perspective ahollow graphite lace and FIG. 2 I

shows a section of a part of a device for vapor-coating according to theinvention. FIG. 3 shows a section of a device for vapor-coatingaccording to the invention.

Example 1.

Two copper wires 2, thickness 0.5 mm., length 320 mm., are slid into ahollow graphite lace 1, length 400 mm., outside diameter approximately 2mm., inside diameter approximately 1 mm., as shown on an exaggeratedscale in FIG. 1 of the drawing, so that on either side approximately 40mm. of the lace is free from metal.

This element is arranged horizontally in a vacuum and heated by acurrent of 13a. With an initial voltage of 20v. For maintaining thiscurrent which is necessary for maintaining a vaporization temperature ofapproximately 2000 C., the voltage must gradually be increased toapproximately 90v.

The copper wires melt. The molten copper does not wet the graphiteelement but an even vaporization of copper around the element isobtained.

In this manner copper mirrors with an even thickness of l p. aredeposited in approximately 20 min. on glass substrates which arearranged at a distance of 80 mm. from the element. The graphite lace wasa material sold by Morganite Research and Development Ltd. asgraphitecord grade 6301 G according to the manufacturer this is aplaited cord about 2 mm. diameter. The electrical resistance is0.45-0.69Qper cm. at room temperature, falling to about half this valueat l50018000E- C. The weight of the cord' is approximately 1.3

gms./m.

I Example 2 A graphite lace as described in example 1 is filled with 1.7gms. of germanium grains over a length of 300 mm.

By heating in a vacuum, the germanium which does not wet the graphitelace is caused to melt and vaporize in a vacuum.

This is carried out in a horizontal setup with a current of 12a. at avoltage increasing from 50 to 13 v. as a result of which a vaporizationtemperature of approximately 1700 C. is reached.

Even germanium mirrors, thickness 2.2. microns, are obtained in thismanner in approximately minutes on glass substrates which are arrangedat a distance of 80 mm. In quite an analogous manner 3 has also beenprocessed to mirrors.

Example 3 A densely braided graphite lace 3, length 444 mm., as shown inFIG. 2, is threaded through an aperture 4 in the bottom of a crucible 5and secured there by means of a knot 6. Aluminum 7 is provided in thecrucible 5 and is kept in the molten state at approximately 800 C. Asshown in FIG. 3 the crucible 5 is positioned in an oven 8. The lace 3 isbrought under a weak tensile stress by the leaf springs 14 to which thelace 3 is connected through member 11 having an aperture 15 in whichlace 3 is secured by means of a knot 12 and the means 13 for holdingmember 11. Substrate holders 10 are arranged around the lace 3.Electrical connections are made with spring 9 which is arranged betweenthe crucible 5 and the inner wall of the electric oven 8 and the holdingmeans 13. The device as shown in FIG. 3 is positioned in a vacuumchamber (not shown). The chamber is evacuated. The mass 7 consisting ofaluminum is heated to a temperature of 800 C. by means of the oven 8.The lace 3 is heated to approximately 2000 C. with a current of 27a.with an initial voltage difference of 80v. between the springs 9 and theholding member B. Alu'mihum is sucked up from the crucible and reactswith the graphite while forming aluminum carbide. At the same time freealuminum is taken up in the element. With a current passage of 100a. anda voltage of 8v. a temperature of approximately 1200 C. is generated. Onsubstrates which are arranged at a distance of 40 mm., aluminum mirrors,thickness 20 ,u., are vapor-deposited in this manner in 20 minutes.

During the vaporization process, the aluminum in the element isreplenished by a capillary sucking from the crucible. By supplyingaluminum wire to the Cl'UCllJ e the process can be Example 4 A rod oftitanium, length 250 mm., thickness 2 mm., is slid into a hollowgraphite lace as described in example 1. Heating is carried out in avacuum by the passage of current.

At 60a. reaction occurs while forming titanium carbide, while a part ofthe titanium remains in the tubular element in a free state.

With a current of 60a. at 27 volt, the free titanium is deposited onglass substrates which are arranged at a distance of mm. Titaniummirrors, thickness 1 u, are then obtained in approximately 10 minutes.

Example 5 In a manner quite analogous to that described in example 4 agraphite lace is converted into a zirconium carbide element, thequantity of zirconium being chosen to be so that only a small excess ofthis metal remains.

The resulting hollow zirconium carbide element is filled with chromiumin powder form and then heated in a vacuum to approximately 1200 C. by acurrent of 50a. with 20v.

Chromium mirrors, thickness approximately 1 [.L, are then obtained in 25minutes on substrates which are arranged at a distance of 70 mm.

In the same manner mirrors of cobalt are realized.

What I claim is:

l. A method of coating substrates by vapor-deposition from the surfaceof a vaporizer element built up from graphite and heated by the passageof current, wherein the substance to be vapor-deposited is taken up in avaporizer element consisting of a lacelike body which is built up fromgraphite yarn and is then heated at a temperature at which the saidsubstance vaporizes.

2. A method as claimed in claim 1, wherein the elementary strands of thegraphite yarn have a thickness of under 20 u.

3. A method as claimed in claim 1, characterized in that thevaporization process is carried out by means of a porous lacelikevaporizer element which is provided with a coaxial cavity.

4. A method as claimed in claim 1, characterized in that the substanceto be vapor-deposited is replenished during vaporization.

5. A method as claimed in claim 1, characterized in that the graphite isconverted into a metal carbide at least superficially. I

6. A method as claimed in claim 5, characterized in that the graphite isconverted into zirconium carbide.

2. A method as claimed in claim 1, wherein the elementary strands of thegraphite yarn have a thickness of under 20 Mu .
 3. A method as claimedin claim 1, characterized in that the vaporiZation process is carriedout by means of a porous lacelike vaporizer element which is providedwith a coaxial cavity.
 4. A method as claimed in claim 1, characterizedin that the substance to be vapor-deposited is replenished duringvaporization.
 5. A method as claimed in claim 1, characterized in thatthe graphite is converted into a metal carbide at least superficially.6. A method as claimed in claim 5, characterized in that the graphite isconverted into zirconium carbide.